Real key pattern is at.<domain>.<name> (e.g. at.ios.only, at.promo).
The fc. prefix in the BFF README and API_Mobile_App.md was incorrect.
- API_Mobile_App.md: updated request keys, response raw examples,
flag table, and Swift usage examples to at. prefix
- API_Mobile_App.md: clarified that app uses raw["at.key"] not
simplified flags object (FeatureFlagsState.isEnabled reads raw)
- feature-control-bff/README.md: updated DEFAULTS_JSON example,
request keys, response flags/raw examples, added note about raw usage
